The helical TC sounds like a great idea. I might have to have a hunt for one in the future

Look for the Vivitar or the Teleplus ones. One is a rebranded version of the other. With that type, the front of the teleconverter extends out as you turn the rubber grip. As a 2x teleconverter they are pretty sharp too so the housing that holds the elements is worth keeping. I have two in Nikon mount, one I kept the glass in and the other I removed it, paid around £15 to £30 but they can go for much higher, seen them at £100 for example. There other types of helical macro focusing teleconverters (Elicar Macronet and Panagor) where all the movement is internal and so these don't work as a helical focuser as nothing actually extends.